A blind system typically includes a screen (which refers to a flexible, semi-rigid or rigid sheet of material such as canvas, fabric, mesh or a panel) with one end secured to and/or gathered by a rotatable roller, and another end secured to a draw bar. A screen may also be referred to as a curtain, awning or shade. The roller may be secured to a supporting frame, structure or surface. The roller rotates in one direction to extend the screen over an area or opening to be covered (e.g. a wall or window), and rotates in the other direction to retract the screen. The sides of the screen may hang freely when extended.
When a blind system is used in windy or air turbulent environments (such as for covering an opening window, skylight or in outdoor environments), the turbulence may cause the screen to vibrate and cause damage to the screen itself or to surrounding items. For example, a screen hanging freely over a window may flap violently when a strong gust of wind enters or passes the window. The screen and draw bar may strike or damage items and injure people in close proximity to the window.
Several attempts have been made to address this issue. Australian patent no. 2002300183 describes a track guided blind system where the edges of a screen extend through a slot of a respective guide track and then into an internal cavity of the guide track. The edge of the screen receives a cord (or rope) to make the edge larger in diameter than the size of the slot. The screen can therefore slide along the guide track with its edges securely retained in the cavity. The screen is installed taut so that there would be little or no flapping of the screen in windy conditions. The ends of the draw bar are adapted to fit into (and slide along) the slot of the guide track. There are several problems with this approach. The draw bar has much greater thickness than the screen. If the slot is sufficiently large to receive an end of the draw bar, the screen will be able to move within the slot (i.e. from one edge of the slot to the other) and can therefore flap in windy conditions. However, if the slot is sufficiently small so that the edges of the slot are close to the surface of the screen, then the end of the draw bar will need to be made very thin, which would be structurally weak and therefore prone to damage. Further, when the screen is stretched, there will be greater frictional resistance between the edge of the screen and the inside of the internal cavity, thus making it harder to extend and retract the screen along the guide track. Also, stretching the screen for extended periods causes the screen to gradually lose its elastic characteristics, and therefore be less effective in resisting flapping in windy conditions (since the guide tracks will be fixed in position when installed).
Australian patent no. 598354 describes another blind system where the edge of a screen extends through a slot of a guide track and adapted to be securely received inside a longitudinal channel of the guide track. The blind system has cushioning materials for biasing the guide track away from the screen and thus keeping the screen tightly stretched. When the blind system is initially installed, the guide tracks are positioned so as to stretch the screen, and in this configuration, the cushioning materials will be slightly compressed. Over time, as slack develops in the screen, the cushioning materials decompress to provide a biasing force that urges the guide tracks away from the screen thereby maintaining tension in the screen. However, the need to have cushioning materials increases the mechanical complexity of the system.
It is therefore desired to address one or more of the above problems, or to at least provide a useful alternative.